The expression “on-line generation” of data during a reaction is used herein to denote generation of the data sufficiently rapidly that the data is available essentially instantaneously for use during the reaction. The expression “generation of data in on-line fashion” during a reaction is used synonymously with the expression on-line generation of data during a reaction. Generation of data from a laboratory test (on at least one substance employed or generated in the reaction) is not considered “on-line generation” of data during the reaction, if the laboratory test consumes so much time that parameters of the reaction may change significantly during the test. It is contemplated that on-line generation of data can include the use of a previously generated database that may have been generated in any of a variety of ways including time-consuming laboratory tests.
With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. The most recently produced quantity typically undergoes mixing with previously produced quantities of the product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
Throughout this disclosure, the expression “diluent” (or “condensable diluent” or “condensable diluent gas”) denotes condensable gas (or a mixture of condensable gases) present in a polymerization reactor with polymer resin being produced. The diluent is condensable at the temperatures encountered in the process heat exchanger. Examples of diluents include induced condensing agents (ICAs), comonomers, isomers of comonomers, and combinations thereof.
The expression “dry polymer resin” (or “dry version” of polymer resin) is used herein to denote polymer resin that does not contain substantial amounts of dissolved gas. An example of dry polymer resin is polymer that had been previously produced in a polymerization reactor and then purged to eliminate all (or substantially all) unreacted comonomers and ICAs that had been dissolved in the polymer at the time of production. As will be discussed herein, a dry version of polymer resin has significantly different melting behavior than would the same polymer resin if it were in the presence of a significant amount of condensable diluent gas and comonomer.
The expression polyethylene denotes a polymer of ethylene and optionally one or more C3-C10 α-olefins while the expression polyolefin denotes a polymer of one or more C2-C10 α-olefins.
Throughout this disclosure, the abbreviation “MI” denotes melt index.
One commonly used method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed (fluid bed) reactor, during operation to produce polyolefins by polymerization, contains a fluidized dense-phase bed including a mixture of reaction gas, polymer (resin) particles, catalyst, and (optionally) catalyst modifiers. Typically, any of several process control variables can be controlled to cause the reaction product to have desired characteristics.
Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of a catalyst. This gaseous stream is withdrawn from the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is removed in another part of the cycle by a cooling system external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the resin and gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Further, heat removal is important to prevent excessive stickiness of polymer particles that if left unchecked, may result in loss of fluidization or agglomeration of the sticky particles which may lead to formation of chunks or sheets of polymer that cannot be removed as product. Further, such chunks or sheets may fall onto the distributor plate causing impaired fluidization, and in many cases forcing a reactor shutdown. Prevention of such stickiness has been accomplished by controlling the temperature of the fluid bed to a temperature below the fusion or sintering temperature of the polymer particles. Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to agglomeration or stickiness, which in turn can, if left unchecked, may lead to the above conditions.
It is understood that the amount of polymer produced in a fluidized bed polymerization process is directly related to the amount of heat that can be withdrawn from the fluidized bed reaction zone since the exothermic heat generated by the reaction is directly proportional to the rate of polymer production. In steady state operation of the reaction process, the rate of heat removal from the fluidized bed must equal the rate of rate of heat generation, such that the bed temperature remains constant. Conventionally, heat has been removed from the fluidized bed by cooling the gas recycle stream in a heat exchanger external to the reactor.
A requirement of a fluidized bed process is that the velocity of the gaseous recycle stream be sufficient to maintain the reaction zone in a fluidized state. In a conventional fluidized bed polymerization process, the amount of fluid circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate mixing of the solids in the fluidized bed. The excess velocity provides additional gas flow to (and through) the fluid bed for additional cooling capacity and more intensive mixing of the reactor bed. However, to prevent excessive entrainment of solids in a gaseous stream withdrawn from the fluidized bed, the velocity of the gaseous stream must be regulated.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream without causing problems of polymer agglomeration or plugging of the reactor system. The dew point of the recycle stream is that temperature at which liquid condensate first begins to form in the gaseous recycle stream. The dew point can be calculated knowing the gas composition and is thermodynamically defined using an equation of state.
Contrary to this belief, as suggested by Jenkins, et al. in U.S. Pat. No. 4,543,399 and related U.S. Pat. No. 4,588,790, a recycle stream can be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream. The resulting stream containing entrained liquid is then returned to the reactor without causing the aforementioned agglomeration and/or plugging phenomena (which had been expected prior to Jenkins). The process of purposefully condensing a portion of the recycle stream is known in the industry as “condensed mode” operation in a gas phase polymerization process.
The above-cited U.S. patents to Jenkins et al. suggest that when a recycle stream temperature is lowered to a point below its dew point in “condensed mode” operation, an increase in polymer production is possible, as compared to production in a non-condensing mode because of increased cooling capacity. Consequently, a substantial increase in space-time yield, the amount of polymer production in a given reactor volume, can be achieved by condensed mode operation with little or no change in product properties.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture with solids contained in both of these phases. The liquid phase of this two-phase gas/liquid mixture in “condensed mode” operation remains entrained or suspended in the gas phase of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. In the process described by Jenkins, et al., vaporization occurs when the two-phase mixture enters the fluidized bed, with the (warmer) resin providing the required heat of vaporization. The vaporization thus provides an additional means of extracting heat of reaction from the fluidized bed. The heat removal capacity is further enhanced in condensed mode operation by the lower gas temperatures of the gas stream entering the fluidized bed. Both of these factors increase the overall heat removal capability of the system and thereby enable higher space-time yields (higher reactor production rates per unit volume of the fluidized bed).
Jenkins, et al. illustrate the difficulty and complexity of such reactor control in general, and of trying to extend the stable operating zone to optimize the space time yield in a gas phase reactor, especially when operating in condensed mode.
The cooling capacity of recycle gas can be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. One option described is to add non-polymerizing, non-reactive materials to the reactor, which are condensable at the temperatures encountered in the process heat exchanger. Such non-reactive, condensable materials are collectively known as induced condensing agents (ICAs). Increasing concentrations of ICA in the reactor causes corresponding increases in the dew point temperature of the reactor gas, which promotes higher levels of condensing for higher (heat transfer limited) production rates from the reactor. Suitable ICA materials are selected based on their specific heat and boiling point properties. In particular, an ICA compound is selected such that a relatively high portion of the material is condensed at the cooling water temperatures available in polymer production plants, which are typically 20-40° C. ICA materials include hexane, isohexane, pentane, isopentane, butane, isobutane and other hydrocarbon compounds that are similarly non-reactive in the polymerization process.
U.S. Pat. No. 5,352,749, to DeChellis et al, teaches, among other things, that there are limits to the concentrations of condensable gases, whether ICA materials, comonomers or combinations thereof, that can be tolerated in the reaction system. Above certain limiting concentrations, the condensable gases can cause a sudden loss of fluidization in the reactor, and a consequent loss in ability to control the temperature in the fluid bed. U.S. Pat. Nos. 5,352,749, 5,405,922 and 5,436,304, suggest upper limits of ICA in the reactor, depending on the type of polymer being produced. The authors characterized the upper limit of condensable materials by tracking the ratio of fluidized bulk density to settled bulk density. As the concentration of isopentane (ICA) was increased in an otherwise steady-state reaction, they found that the bulk density ratio steadily decreased. When the concentration of isopentane was sufficiently high, corresponding to a bulk density ratio of 0.59, they found that fluidization in the reactor was lost. They, therefore, determined that this ratio (0.59) represented a limiting value below which a reactor would cease functioning due to loss of fluidization.
As described in PCT Application Publication Number WO 2005/113615(A2) and U.S. Pat. No. 7,122,607, attempts to operate polymerization reactors with excessive ICA concentrations cause polymer particles suspended in the fluid bed to become cohesive or “sticky” and in some cases cause the fluid bed to solidify in the form of a large chunk. This stickiness problem is characterized by undesirable changes in fluidization and mixing in the fluid bed, which if left unchecked, may develop into a reactor discontinuity event, such as sheeting in the straight sided reaction section, sheeting in the dome of such a reactor or chunking, any of which can lead to reactor shut-downs, which in large scale reactors are expensive. These solid masses (sheets or chunks) of polymer eventually become dislodged from the walls and fall into the reaction section and settle on the distributor plate, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning. The term “discontinuity event” is used to describe a disruption in the continuous operation of a polymerization reactor caused by sheeting, chunking or distributor plate fouling. The terms “sheeting and/or chunking” while used synonymously herein, may describe different manifestations of problems caused by excessive polymer stickiness in the fluid bed. In either manifestation (sheeting or chucking) the excessive polymer stickiness can lead directly to a reactor discontinuity event with the associated loss production.
WO 2005/113615(A2) and U.S. Pat. No. 7,122,607 disclose a “critical temperature” (sometimes denoted herein as “CT”) of a resin-producing polymerization reaction in a gas phase fluid-bed reactor and use of this critical temperature to control the reactor. The CT is a property of the specific polymer (e.g., polyolefin) produced by the reaction and is a temperature in the fluid bed below which the polymer cannot become sticky regardless of the concentration of condensable diluent(s) in the reactor. Thus, if the reactor were operated with a temperature equal to or less than the CT to produce the polymer in the fluid bed, it would be impossible for the polymer to become sticky even under conditions of maximum depression of the polymer's dry sticking temperature (where the actual amount of depression of the dry sticking temperature would depend on the actual concentration of condensable diluent(s) in the reactor). CT varies with the characteristics of a polymer (e.g., density and MI) but not with temperature and other reaction conditions of the polymerization reaction which produces the polymer.
The “dry sticking temperature” of a polymer in a fluid bed reactor is defined in U.S. Pat. No. 7,122,607 as the temperature at which agglomeration or fouling on any surface of the reactor vessel begins to occur, or the temperature at which there is at least a 50% drop in bandwidth of the bed DP reading, whichever is less, with the reactor operating at normal pressure and gas velocity but in the presence of substantially pure nitrogen rather than the gas components actually present with the polymer during the reaction (where “bed Dreading” denotes measured pressure difference between the bottom and top of the fluid bed). “Melting point depression” is the temperature by which the dry sticking temperature of a polymer in a polymerization reactor (or a temperature assumed to be at least substantially equal thereto, e.g., the melting point of a dry version of the polymer) is reduced by the presence of condensable diluent (ICA and comonomer) used during the reaction.
The CT disclosed in U.S. Pat. No. 7,122,607 is the polymer's dry sticking temperature minus the largest melting point depression that could occur due to the presence of condensable diluent(s) in the reactor. The difference between dry and fully immersed (liquid) Differential Scanning Calorimeter (“DSC”) peak melting temperatures for the polymer is taken to be the maximum melting point depression, with the DSC peak melting temperature of the dry polymer assumed to correspond to the polymer's dry sticking temperature. The CT disclosed in U.S. Pat. No. 7,122,607 is typically not the same temperature as the temperature dMIT=ΔMIT defined in the MIT application discussed below. The value of dMIT depends on the concentration of condensable diluent(s) in a polymerization reactor during production of a polymer, and thus can vary as a function of time during the reaction as diluent concentration changes. Depending on the current value of dMIT, the reaction may be subject to a high or low risk of occurrence of reactor sheeting or another discontinuity event. In contrast, the CT for a polymer is a limiting value that bounds the set of all the possible dMIT values that can exist during production of the polymer.
WO 2005/113615(A2) and U.S. Pat. No. 7,122,607 do not teach a method for on-line monitoring of a polymerization reaction including by monitoring the reaction to generate reaction parameter measurements and determining (in on-line fashion) the reaction's CT (or any other parameter indicative of onset or degree of stickiness) from these measurements. Rather, determination of the reaction's CT in accordance with the teaching of these references would require laboratory tests that could not be performed sufficiently rapidly during the reaction so that the CT would be available essentially instantaneously for use during the reaction.
Two articles by Process Analysis & Automation Limited (PAA), entitled “Agglomeration Detection by Acoustic Emission,” PAA Application note: 2002/111 (© 2000) and “Acoustic Emission Technology—a New Sensing Technique for Optimising Polyolefin Production” (©2000), suggest process control in fluid bed production of polyolefins utilizing acoustic emission sensors located at various positions on the reactor and recycle piping. These publications purport to solve the problem of detecting large polymer agglomerates in a reactor, such as chunks or sheets, rather than detecting stickiness of the resin particles, and provide only one specific example, showing the detection of a chunk of approximately 1.5 meters in diameter within a commercial fluid bed reactor. There is no mention of the detection of polymer stickiness or cohesiveness. In effect, the PAA documents describe the detection of agglomerates after they have been formed in the reactor, rather than detection of resin stickiness that, if left unchecked, could lead to the formation of the agglomerates.
PCT Application Publication Number WO 03/051929 describes the use of mathematical chaos theory to detect the onset and presence of sheeting in a fluid bed reactor. Signals from a range of instruments, including acoustic emission sensors, differential pressure sensors, static sensors, and wall temperature sensors are filtered by certain specified methods to construct a “time-series” of data, which is then processed by methods of non-linear dynamics herein referred to as chaos theory and compared to data from a control reactor running without sheeting. The onset of sheeting is indicated by an increase in mean “cycle time” (relative to a baseline, control reactor), usually with a concurrent decrease in the “mean deviation” of the time-series. Alternatively, the onset of sheeting is indicated by a decrease in the mathematical “entropy” of the time-series data, as compared to a similar reactor running without sheeting. (The terms “time-series”, “cycle time”, “mean deviation”, and “entropy” here refer to calculated parameters defined by chaos theory.) This reference does not disclose processing of sensor readings (without recourse to the complexities involved with chaos theory) to generate data indicative of conditions at which the resin in a reactor is predicted to become sticky, or any method allowing safe operation of a polymerization reactor near its limit of ultimate cooling capacity for maximum production rates.
Adding to the complexity of control of stickiness while using ICAs, different polymer products vary widely in their ability to tolerate ICA materials, some having a relatively high tolerance (expressed in partial pressure of the ICA in the reactor), e.g. 50 psia, while other polymers may tolerate as little as 5 psia. In these latter polymers, the heat transfer limited production rates under similar conditions are substantially lower. Polymers which possess a more uniform comonomer composition distribution are known to have a higher tolerance to the partial pressure of the ICA in the reactor. Metallocene catalyst produced polymers are a good example of polymers with such a more uniform comonomer composition. However, at some point even these metallocene produced polymers reach a limiting ICA concentration that induces stickiness. The limiting ICA concentration depends on several factors in addition to the polymer type, including reactor temperature, comonomer type and concentration. Further, with the effect of temperature, ICA level and comonomer levels all affecting on the onset of stickiness, determining the point at which sticking begins to occur has heretofore been difficult.
Even within the constraints of conventional, safe operation, control of such reactors is complex adding further to the difficulty and uncertainty of experimentation if one wishes to find new and improved operating conditions that might result in higher production rates. Large-scale gas phase plants are expensive and highly productive. Risks associated with experimentation in such plants are high because downtime is costly. Therefore it is difficult to explore design and operating boundaries experimentally in view of the costs and risks.
It would be desirable to provide a method of determining a stable operating condition for gas phase fluid bed polymerization, especially for condensed mode operation, to facilitate optimum design of the plant and determination of desirable process conditions for optimum or maximum production rates in a given plant design.
It would also be desirable to have a mechanism in commercial gas-phase reactors to detect the onset of stickiness that is a better or earlier indicator of the onset of stickiness than are conventional techniques (e.g., monitoring the fluidized bulk density as described in U.S. Pat. No. 5,352,749). Such a mechanism would allow the operators to determine when conditions of limiting stickiness are being approached, and enable them to take corrective action before discontinuity events (such as sheeting and chunking) occur, while keeping the reactors at or near conditions of maximum ICA concentration, permitting higher production rates with substantially less risk.
U.S. Patent Application Publication No. 2007/0060721 A1, published on Mar. 15, 2007, and entitled “Method for Operating a Gas-Phase Reactor at or Near Maximum Production Rates While Controlling Polymer Stickiness,” by Michael E. Muhle and Robert O. Hagerty, filed as application Ser. No. 11/227,710 on Sep. 14, 2005, discloses monitoring of resin stickiness (during operation of a polymerization reactor) by generating a time series of readings of acoustic emissions of the contents of the reactor using acoustic emission sensors. The method includes a preliminary step of generating acoustic emission measurements during steady state operation of the reactor to produce a polymer resin by polymerization. Additional acoustic emission measurements are then generated during operation of the reactor and the latter measurements are processed to determine whether the measured emissions deviate from acoustic emissions indicative of steady state reactor operation. Such deviation is treated as an indication of onset of excessive stickiness of polymer particles in the reactor, in response to which corrective action can be taken (e.g., ICA and/or monomer levels and/or reactor temperature can be adjusted). However, this reference does not teach determination of a reference temperature above which polymer resin in a reactor is predicted to become sticky.
More specifically, above-referenced U.S. Application Publication No. 2007/0060721 teaches detecting onset of excessive stickiness (of polymer within a fluid bed reactor) by monitoring a running average of readings from one or more acoustic emission sensors positioned adjacent to the fluid bed. The application teaches that the acoustic emission sensors can be located or mounted on the exterior of the reactor adjacent to (but outside) the fluid bed, and that in typical cases in which the reactor has a cylindrical portion above a distributor plate and below a conical top section, the acoustic emission sensors can be positioned along any part of the cylindrical portion from the top of the distributor plate to the junction of the cylindrical wall with the conical section (e.g., the sensors can be positioned more than 0.05, 0.1, 0.15, 0.2 or 0.25 reactor diameters above the distributor plate, and/or more than 0.05, 0.1, 0.15, 0.2 or 0.25 reactor diameters below the cylindrical-conical junction, where, although the distributor plate is on the inside of the reactor, the sensors can be positioned above the level of the distributor plate but on the exterior of the reactor). Any number (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, or more) of acoustic emission sensors can be positioned along the cylindrical section of the reactor.
In the method described in U.S. Application Publication No. 2007/0060721, the running average of readings is calculated using a “moving time window” averaging method. The average is defined as the sum of n individual readings in a time window divided by n:
      X    _    =                    ∑                  i          =          1                n            ⁢              X        i              n  where X is the current value of the running average and Xi is an individual reading. U.S. Publication No. 2007/0060721 teaches that the n individual sample points are preferably collected at equally spaced time intervals within the window, that suitable time windows for the time window averaging are 0.01 to 1000 seconds, or 0.1-750 seconds, or 1 to 500 seconds, and that a significant increase in resin stickiness is indicated by a “significant” decrease in the running average signal, defined as a decrease of one or more standard deviations of that signal. The standard deviation (“s”) is computed by the formula:
      s    2    =                    ∑                  i          =          1                n            ⁢              (                              X            i                    -                      X            _                          )                    (              n        -        1            )      where Xi is an individual reading of an acoustic emission sensor within the window, n is the total number of observations, and X is the running average of the acoustic emission signal, described above. The number of sample points used in the calculation of standard deviation is equal to the number of sample points involved in the calculation of the running average. If, for example, the time window for computing the running average is 60 seconds, and the sampling frequency is 10 points per second, then n is equal to 600. The sample points Xi used in the calculation of standard deviation may be the same as those used in the running average calculation.
U.S. Publication No. 2007/0060721 teaches that suitable sampling frequencies for the acoustic emission sensors for use in the running average and standard deviation calculations may be from 0.01 to 1000 samples per second, or 0.1-750 seconds, or from 1 to 500 samples per second, and that the total number of samples n involved in the calculations (equal to the product of the window width and the sampling frequency) should be from 10 to 100,000 or from 50 to 10,000, and that increasing stickiness of reactor contents is indicated by a decreasing level of acoustic emissions in the fluidized portion of the bed (i.e., by a “quieter” bed, in terms of its acoustic emissions signal).
U.S. Publication No. 2007/0060721 also teaches that individual grades of polymer produced in gas-phase reactors under condensed mode conditions are subject to different operating conditions and will tolerate different levels of ICA(s) and/or comonomer(s), and/or temperature and therefore will have different limiting stickiness thresholds or points (due to the effect of the different molecular weights and comonomer incorporation levels of the different grades), and thus that to determine an expanded operating window (to increase production rates) by use of acoustic emission sensor(s), the operators of the process, for each given grade, should first run the process at steady state in a safe condition with optimum production conditions, and record the acoustic emissions under each such set of steady state conditions. Then, to increase production rate, reactor conditions are changed (e.g., by increasing the catalyst feed rate and/or increasing ICA concentration level and/or increasing reactor temperature) and the acoustic emissions are again monitored. When the measured acoustic emissions drop by a predetermined number of standard deviations (e.g., by 0.1, or 1, or 2, or more standard deviations) below the corresponding steady state, “safe” mode level of acoustic emissions (e.g., when the reactor is determined to be “quieter” by a sufficient degree as determined by the processed acoustic emission sensor measurements), the operators can take corrective action.
As described in U.S. Publication No. 2007/0060721, each acoustic emission sensor is essentially a small microphone that can detect and amplify high frequency (ultrasonic) sound waves. The sensor typically utilizes a piezoelectric transducer to detect the acoustic signal generated by the impact of resin particles on the walls surrounding a flowing stream (the fluid bed section of a reactor). The acoustic signal is normally measured in the ultrasonic range. The sensors may be narrow bandwidth piezo-electric sensors with local pre-amplifiers producing an industry standard gain of 40 decibels (dB), where 0 dB equates to a 1 microvolt output from a sensor. The pre-amplifier output can be further amplified using a series of signal amplifiers with a range of 0 to 48 dB to produce measurable signals in the range of 1 to 10 volts. The latter signals can be narrow-band filtered around a center frequency of 190 kHz using a 100-350 kHz band pass filter, and then further conditioned (e.g., in a root mean square filter) to produce an output proportional to lower frequency variations (typically in the audible range of 0 to 20 kHz) in the envelope of the narrow-band filtered acoustic emission signal.
U.S. Patent Provisional Applications No. 60/842,747 (“MRT application”) and 60/842,719 (“MIT application”), both filed on Sep. 7, 2006, describe methods for detecting conditions indicative of imminent occurrence of sheeting during polymerization reactions in fluid bed polymerization reactors, and preferably also controlling the reactions to prevent the occurrence of sheeting.
The MRT application describes a method including of the steps of: monitoring a polymerization reaction which produces a polymer resin in a fluid bed reactor; and in response to data indicative of at least one monitored parameter of the reaction (and a dry melt reference temperature that is characteristic of melting behavior of a dry version of the polymer resin), determining, in on-line fashion, a reduced melt reference temperature characteristic of the melting behavior of the polymer resin as it exists in the reactor. The reduced melt reference temperature (MRTR) is at least substantially equal to the difference between the dry melt reference temperature and a melt reference temperature depression value, “D,” where D is a temperature by which the dry melt reference temperature is depressed by the presence of diluent that is present with the resin in the reactor. The method optionally also includes the steps of determining a stickiness control parameter from the reduced melt reference temperature, and controlling the reaction in response to the stickiness control parameter. A melt reference temperature depression model is used to determine, in on-line fashion, from the data indicative of at least one monitored parameter of the reaction and the dry melt reference temperature value, a reduced melt reference temperature for the polymer resin in the presence of the at least one condensable diluent gas. The melt reference temperature depression model predicts the amount by which the dry melt reference temperature is reduced by the presence with the resin of the condensable diluent gas (e.g., ICA, comonomer, and isomer(s) of at least one comonomer) present with the resin in the reactor during the reaction.
The MIT application describes a specific implementation of the MRT method, including the steps of:                (a) during a polymerization reaction in a fluid bed reactor which produces a polymer resin, measuring parameters of the reaction including at least reactor temperature, at least one resin property of the polymer resin, and concentration of at least one condensable diluent gas in the reactor;        (b) determining from the at least one resin property, using a predetermined correlation, a dry melt initiation temperature of a dry version of the polymer resin; and        (c) during the reaction, using a melt initiation temperature depression model to determine, in on-line fashion from at least some of the parameters measured in step (a) and the dry melt initiation temperature value, a reduced melt initiation temperature for the polymer resin in the presence of the at least one condensable diluent gas, said melt initiation temperature depression model identifying an estimated degree of depression of the dry melt initiation temperature due to presence of at least one diluent with the polymer resin. In typical embodiments, the melt initiation temperature depression model implements the well-known Flory melt depression equation; and the method optionally also includes the step of:        (d) determining in on-line fashion a temperature value indicative of resin stickiness in the reactor, from the reduced melt initiation temperature determined in step (c) and a current value of the reactor temperature. Typically, the temperature value generated in step (d) is a temperature value dMIT that is at least substantially equal to Trx−MITR, where Trx is the current value of reactor temperature, and MITR is the reduced melt initiation temperature determined in step (c). The temperature value indicative of resin stickiness determined in this manner (dMIT) may be used as a parameter to control resin stickiness in the fluid bed.        
The expression “melt reference temperature depression model” is used herein in the same broad sense in which it is used in the MRT application, and the expression “melt initiation temperature depression model” is used herein in the same sense in which it is used in the MIT application. Each melt initiation temperature depression model is a member of the broader class of “melt reference temperature depression models,” so that the set of all melt initiation temperature depression models is a subset of the set of all melt reference temperature depression models.
Until the present invention, it was not known how to perform on-line determination or detection of the onset or degree of resin stickiness in a reactor from acoustic data, generated in on-line fashion during a polymerization reaction and indicative of acoustic conditions in the reactor, without performing a statistical analysis of the acoustic data (e.g., as described above-discussed US Application Publication No. 2007/0060721). Nor had it been known how to monitor a polymer resin-producing polymerization reaction in a fluid bed reactor to generate (in on-line fashion) acoustic data indicative of an acoustic condition in the reactor, and to control the reaction (in on-line fashion) in response to the acoustic data in an effort to prevent occurrence of a discontinuity event.
Use of acoustic emission (“AE”) data to monitor resin stickiness (as described in US Publication No. 2007/0060721) can reliably predict trends (e.g., of increasing or decreasing degree of stickiness) in the AE data. However, this type of method is seriously limited in its ability to give a precise, quantitative prediction of a stickiness limit (a limiting value of a stickiness control parameter determined from the acoustic emission data) because it is difficult to establish a reliable baseline or reference value of a stickiness control parameter from acoustic emission data. Because of this limitation, it is difficult to establish from the acoustic emission data a reliable, quantitative value for a stickiness limit.
US Publication No. 2007/0060721 suggests that the required reference values of a stickiness control parameter are acoustic emission readings when the bed is fluidized at non-sticking conditions, ideally with only nitrogen or other non-soluble gas in the system. In practice, a range of reference values would be required, corresponding to the range of temperatures and resin products to be present in the reactor. Obtaining this data in a commercial reactor system would be time-consuming and expensive. Further complicating matters is the possibility that these reference values (once obtained) could change with time due to instrument drift or changes in the acoustic coupling of the instruments to the reactor wall. The net result is that an acoustic emission system is not sufficiently reliable for quantitative predictions of stickiness limits.
The inventors have recognized that due to the potential for error in measurements of reaction parameters during polymerization reaction monitoring of the types described in the MRT and MIT applications, and the difficulties in establishing the required reference values for polymerization reaction monitoring using acoustic emission data, a need exists for a more reliable method of determining stickiness limits in a gas phase polymerization reactor.
More specifically, the inventors have recognized that the accuracy of dMIT values generated as described in the MIT application depend on the accuracy of monitored process data used to generate the dMIT values, and that each type of process data typically used to generate dMIT values (e.g., fluid bed temperature, gas composition, and resin density and melt index) is subject to error. Measured values of resin properties (e.g., density and melt index) are subject to error due to the relatively complicated procedures typically required for their measurement. Measurement of resin density is particularly prone to error.
Measurement of the composition of gas present with the polymer resin (typically carried out with process gas chromatographs or “GCs”) is usually quite accurate in normal operations, but has a relatively high sensitivity to error. Of particular concern is error caused problems in the small sampling lines that are used to take gas from the reactor to the measuring instrument (e.g., a process GC). These lines are known to be prone to blockage by polymer fines and to gas condensation, either of which can lead to substantial errors in measurement. Measurement errors can also be induced by problems with process GCs, such as unexpected shifts in calibration of the instrument, as well as typical reliability issues associated with any complex instrument.
Of all the measured data typically used to generate dMIT values, the measurement of temperature (e.g., bed or skin temperature) is probably the most accurate and reliable. However, even this type of measurement can be subject to error if the sensing element is covered by a coating of resin in the reactor, or if the instrument becomes mechanically damaged.